Percussion drilling involves delivering laser energy to the workpiece through one or more pulses that define the diameter of the hole. Material within the volume of the hole is removed as a combination of vaporized metal and liquid melt, the latter through a process commonly referred to as ‘melt expulsion’.
Laser melted material from the hole is ejected along the sides of the hole wall, driven by the vapor pressure which develops within the hole. The hole diameter is controlled by the beam diameter at the surface of the material and the energy contained in each pulse.
Percussion drilling is an important enabling technology for the aerospace industry as well as for other industries. First, percussion drilling provides higher throughput than trepanning, the other major drilling process. Furthermore, the same technology used for drilling through holes can produce partial depth holes and surface textures (see ‘Laser Texturing Metal Surfaces Using Laser Drilling Technology‘).
Drilling trials have also shown that percussion drilling can produce holes in some crack sensitive alloys with less cracking in the heat affected zone and base metal compared to similar depth and diameter holes produced by trepanning.
The ability to control the dimensional quality of a percussion drilled hole is also key to percussion drilling being used in production.
For this reason, Prima Power Laserdyne Applications Engineers performed experiments aimed at developing laser parameters that produce holes having quality similar to those produced by trepanning in terms of both geometrical features and metallurgical characteristics.
Some results from these experiments are pictured below.
Figures 1 and 2 show the influence of pulse energy on the hole quality in terms of hole diameter and taper for holes drilled at 30 and 90 degrees to the surface. The data shows that hole diameter increases with pulse energy. However, drilling at an angle to the surface produces slightly smaller diameter holes, presumably due to the reduced power density that results from the larger area of the metal surface that interacts with the elongated laser beam.
We also see that the typical taper is less than 2.5% for the all the laser parameters tested. Unlike for hole diameter, there was no clear correlation between pulse energy and taper. In any case, the taper exhibited for these percussion drilled holes is similar to that for trepanned holes.
Figures 3 and 4 document the shape and metallurgical characteristics (recast layer thickness, cracks) for holes at 30 and 90 degrees to the surface.
Average recast layer thickness for holes drilled at 90 degrees is in the range of 40 to 70µm. For holes drilled at 30 degrees to surface, the recast layer thickness varies between 55 to 95µm.
No base metal cracks were observed for holes of either 30 or 90 degrees.
Quality Holes Faster
Throughput with percussion drilling can be further increased using on-the-fly percussion drilling. See the article on SmartPerf™ for information about Prima Power Laserdyne’s implementation of on the fly drilling for cylindrical and linear patterns.
On the fly drilling involves firing laser pulses to drill in a percussion drilling fashion while the workpiece or the laser beam is continuously moved. This allows the part to be processed in a fraction of the time it would take the same component to be drilled by trepanning.
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